All Suns’ Dawn can have from zero to five targets, one for each color.

You can target two blue-and-white multicolored cards by choosing one as the blue card and one as the white card.

You choose the color each target must be when you choose it as a target, so if the card you targeted as a blue card is red when All Suns’ Dawn resolves, the card is an illegal target, even if the spell doesn’t target another red card.